This invention relates to apparatus and methods for aligning mirrors on the ends of a laser discharge tube.
Conventional gas lasers have a mirror at each end of a discharge tube. In order for lasing to occur, the two mirrors must be aligned parallel to one another. The laser output is enhanced by precision in the mirror alignment. This alignment process has been greatly facilitated by the use of an autocollimator. The autocollimator reflects a generated light beam off an object and indicates when the paths of the incident and reflected beams coincide.
In employing an autocollimator to align a laser, the tube is positioned so that its longitudinal axis is aligned with the path of the light beam. The beam is then directed down the length of the laser tube. A first laser mirror is placed at the far end of the laser tube and then oriented until the autocollimator indicates that the beam is being reflected back along the path of transmission. When this occurs, the mirror is normal to the light beam and therefore normal to the longitudinal axis of the tube. When this is indicated, the mirror is firmly fixed in place. A second mirror is then positioned at the near end of the laser tube with its reflective surface toward the tube. In aligning this mirror, the light beam from the autocollimator travels through the mirror substrate and is reflected from the rear surface of the reflective coating on the mirror. Again the mirror is oriented until the autocollimator indicates that the paths of the incident and reflected beams coincide. Both mirrors then are normal to the longitudinal axis of the tube and parallel to one another.
One of the difficulties encountered in aligning the second laser mirror using an autocollimator is that the light must be transmitted through the second mirror substrate. The mirror substrates are not true parallelograms, i.e., the opposite surfaces are not perfectly parallel. In fact, the mirror substrates are generally made with an unknown wedge angle between their faces. Therefore, when the light enters the substrate of the second mirror it is refracted which distorts the reading of the autocollimator so that when an alignment of the mirror is indicated, the mirror is actually slightly misaligned. It therefore becomes necessary to use two autocollimators to determine the wedge angle and compensate for the refraction of the light beam during alignment of the laser mirror. The use of two autocollimators complicates the alignment procedure increasing both the expense and the time required to align a conventional laser tube.